Progressive lens

ABSTRACT

A progressive lens mold, a method of making a progressive lens mold, and progressive lenses made using such mold or method. Back faces of progressive molds are preferentially tapered at their edges to reduce prism effects in lenses made using such molds. The tapering is accomplished by removing more of the back face adjacent the near portion vision zone of the mold versus the back face adjacent to the far vision zone of the molds.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to molds used to cast plastic progressiveeyeglass lenses, and to methods of making such molds. This inventionalso relates to lenses cast using these molds and methods.

2. Description of Related Art

Progressive eyeglass lenses typically include a front face, a back face,a near vision correction zone, a far vision correction zone, and avision transition zone located between the near vision correction zoneand the far vision correction zone. The front faces of these lenses aretypically substantially convex, and back face of these lenses aretypically substantially concave. These lenses are called "progressive"lenses because their power changes gradually and progressively (i.e.,from the power of the near vision correction zone to the power of thefar vision correction zone) in the vision transition zone. These lensesare generally preferred compared to ordinary or blended bifocal lensesbecause wearers of progressive lenses can focus on objects at neardistances (by looking through the near vision correction zone), fardistances (by looking through the far vision correction zone) anddistances between near and far distances (by looking through differentparts of the vision transition zone). Bifocal lenses only allow wearersto focus on objects at near and far distances since bifocal lenses onlyhave a near vision correction zone and a far vision correction zone.

Molds used to cast progressive eyeglass lenses are hereinafter referredto as "progressive molds." These molds typically include a back facewhich is adapted to cast: (1) a near vision correction zone in the lowerportion of a lens when worn, (2) a far vision correction zone in theupper portion of a lens when worn, and (3) a vision transition zonebetween the near and the far vision correction zones. Progressive moldsare typically used to cure lens forming compositions to form lenses.They are typically used in combination with a second "back" companionmold and a gasket to form a mold cavity. In one embodiment theprogressive mold and companion mold seal against opposite sides of thegasket to form the mold cavity. The mold cavity is then filled with acurable liquid composition, and the liquid composition is cured to asolid by exposure of the composition to light or heat.

Plastic eyeglass lenses are typically cured in the form of a lens"blank" which is close to, but generally not identical to, theprescription needed by the lens wearer. This blank must be furthershaped (e.g. cut, beveled, or ground) to closely fit specific customerprescriptions. This final "prescription shaping" may be completed in aretail environment, or a wholesale laboratory environment.

Recently new methods have been developed for preparing lenses by castinglens-forming compositions between glass molds such that the cast lensesfit specific wearer prescriptions. These cast lenses do not require"prescription shaping" since they are cast to meet the finalprescription needs of the wearer. The lens-forming compositions in theseprocesses may be cured by exposure to ultraviolet light or heat, or byany free radical polymerization process. Ultraviolet light processes areadaptable to produce lenses in time periods of less than one hour. As aresult, lenses can be directly cast, without further shaping, to fitcustomer prescriptions and still provide quick service. Some of suchcuring techniques are described in the U.S. patents and co-pendingapplications described below:

co-pending U.S. application entitled "Apparatus and Process For LensCuring and Coating" by Buazza et al., filed the same day as thisapplication, and which is a continuation-in-part of co-pending U.S.application Ser. No. 800,561, filed Dec. 6, 1991, which is acontinuation-in-part of co-pending U.S. application Ser. No. 642,614,filed Jan. 17, 1991, which is a continuation-in-part of co-pending U.S.application Ser. No. 425,371, filed Oct. 26, 1989, which is acontinuation-in-part of Ser. No. 273,428, filed Nov. 18, 1988, now U.S.Pat. No. 4,879,318, which is a continuation-in-part of Ser. No. 021,913,filed Mar. 4, 1987, now abandoned, which is a continuation-in-part ofSer. No. 823,339, filed Jan. 28, 1986, now U.S. Pat. No. 4,728,469.

These patents and co-pending applications are incorporated by reference.In addition, U.S. Pat. No. 4,919,850 also describes methods andapparatus for making plastic lenses, and this patent is alsoincorporated by reference.

Progressive molds may be made using a variety of methods well known inthe art such as grinding. Grinding progressive molds, however, can beunduly expensive and difficult due to the uneven back face of the mold.An alternate method to make progressive molds is a "heat slumping"process. This process is preferred by some manufacturers for preparingprogressive molds because it is relatively less expensive than otherprocesses such as grinding.

When making progressive molds using a heat slumping process, a first orprototype surface may be prepared which is a replica of the front faceof the progressive lens desired. This first surface may be concave orconvex, and typically comprises a ceramic material. After the firstsurface has been prepared, mold material is placed over this firstsurface. Typically this mold material is made of glass and is shaped inthe form of a disk. The mold material and the first surface are thenheated to a high enough temperature such that the mold material "slumps"or molds around the first surface, thereby forming a progressive moldwith a back face substantially similar to the front face of the lensdesired. The resulting mold also has a front face, and side surfacesconnecting the front face to the back face. The mold is then removedfrom the first surface.

Whether made by grinding, heat slumping, or other processes, one problemwith nearly all existing progressive molds is that they tend to castlenses that have "vertical prism" effects. If uncorrected, verticalprism effects cause wearers of such lenses to see images at differentvertical levels than the images really are (i.e., the lenses tend tocause the wearer's vision to shift slightly in the vertical direction).Wearers of lenses with pronounced vertical prism imbalance tend toexperience diplopia (double vision), nausea, and/or suffer headaches.Vertical prism effects are believed to be caused by excessivedifferential thicknesses between the near and far vision correctionzones in progressive lenses. Specifically, the bottom of the lensproximate the near vision zone is thicker than the top of the lensproximate the far vision correction zone. If the amount of thicknessdifference is excessive, it may cause an unacceptably large amount of(base down) vertical prism effects.

Vertical prism effects exist but are relatively easily corrected inprogressive lens blanks. The correction takes place when the lens blanksundergo prescription shaping. The prescription shaping process usuallyinvolves applying a shaping instrument (such as a grinder) to the centerof the concave surface of the lens. To hold the lens blank in place, theconvex surface of the lens blank is attached to a lens "block." Thislens block is attached to the geometric center of the convex surface ofthe lens blank. Any desired prism effect correction is made by insertinga "prism ring" around the block such that it can seat against the lensblank. The prism ring is designed to tilt the lens blank the appropriateamount necessary to increase or decrease vertical prism effects in thelens blank when the lens blank is shaped to prescription. The tiltingcaused by the prism ring also affects the thickness of the edges of theresulting lens. Sometimes practitioners will vary the vertical prismeffects (i.e. by varying the prism ring tilt) in an effort tocosmetically optimize lens edges. This process is well known in the art.

Vertical prism effects are more difficult to treat with lenses that castto prescription since these lenses do not undergo a prescription shapingprocess. Thus vertical prism effects remain in these lenses unless othermeasures are taken to reduce or eliminate them. These measures are thesubject of the invention described herein.

SUMMARY OF THE INVENTION

In a general aspect, the present invention includes a system to reducevertical prism effects in prescription lenses cast using progressivemolds. In another aspect, the invention includes a mold for castingprescription progressive lenses with reduced vertical prism effects. Theinvention also includes progressive lenses made by the system of theinvention.

In a more specific aspect, the invention is directed to a progressivemold disk with a front face, and a back face which is concave and tracksthe convex front face of a lens desired to be cast with the mold. Themold disk is bounded by a generally cylindrical surface which joins thefront and back faces. In addition, the back face has a relatively narrowannular surface or band which extends around its outer edge orperiphery. This annular surface or band is relatively flat and extendsbetween the outer boundary of the concave portion of the back face andthe outer boundary of the back face itself. The width of the band variesaround the periphery of the back face. It is usually widest at theportion of the back face corresponding to the bottom of the near visioncorrection zone of a lens formed by the mold. It is narrower at theportion of the back face corresponding to the top of the far visioncorrection zone of a lens formed by the mold. In general, these twoextreme portions of the band are both wider than the remainingintermediate portions.

A key feature of the mold component of the invention resides in theformation of the annular surface or band around the edge of the backface. This surface or band is formed by tapering the back face of themold at a "correction" angle such that more of the back face near thebottom of the near vision correction zone is removed versus the backface near the top of the far vision correction zone. The surface or bandnear the bottom of the near vision correction zone thus tapers or slopesslightly inward and towards the center of the mold. The surface or bandnear the top of the far vision correction zone tapers or slopes slightlyinward and away from the center of the mold. The angle of taper or slopecorresponds to the correction angle.

An advantage of the invention is that vertical prism effects can becorrected in cast prescription lenses without modifying the gasket orthe molds used to cast the back face of such lenses (i.e. the "backmolds"). As a result, several different prescription progressive lensescan be cast using identical gaskets and back molds. In such case, oftenonly the progressive molds themselves need to be varied to makedifferent progressive lenses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a top view of a progressive mold of theinvention with a front face and a back face. The back face of this moldis facing upwards.

FIG. 2 depicts a side view of the mold shown in FIG. 1.

FIG. 3 depicts a schematic diagram of a mold being formed according tomethod of the invention.

FIG. 4 depicts an enlargement of portion 30 in FIG. 3.

FIG. 5 depicts an enlargement of portion 31 in FIG. 3.

FIGS. 6A and 6B depict lenses made by uncorrected and correctedprogressive molds.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically depicts a progressive mold 2 which is adapted tocast a progressive plastic eyeglass lens according to the method of theinvention. Like all the figures, FIG. 1 is not an exact representationof a progressive mold 2. FIG. 1 is instead only intended to depictvarious important features of the invention. The mold 2 typicallyincludes a back face 5 which includes a section adapted to cast a nearvision correction zone 4, a section adapted to cast a far visioncorrection zone 6, and a section adapted to cast a vision transitionzone 3. As shown in FIGS. 1 and 2, the mold 2 may be disk-shaped,however it is to be understood that other shapes such as ovals, squares,and rectangles are also possible.

The back face 5 of mold 2 typically includes a generally flat surface orband 7 and a substantially concave surface 18. The width of band 7 mayvary around the circumference of mold 2, however it tends to be widernear the bottom 30 and top 32 of the mold 2. Specifically, band 7 mayhave a width 8 when at the bottom 30 and a width 10 when at the top 32.Band 7 is advantageous in that it provides a surface for gaskets to sealagainst the mold, thus forming a mold cavity for lens casting.

FIG. 2 depicts the mold 2 as viewed from the side. The mold 2 typicallyincludes a convex front face 20 with a side surface 22. Side surface 22is drawn to be substantially flat, however it is to be understood thatthis surface 22 may also be in other shapes such as irregular, convex,or concave shapes. The center 21 of the mold 2 may be thinner or thickerthan the thickness of the mold 2 at various radii 23 from the center 21.Although not clearly shown in FIG. 2, it is to be understood that band 7angles inward and toward center 21 at the bottom 30, and inward and awayfrom center 21 at top 32.

FIG. 3 is a schematic drawing depicting the method of the invention. Asdiscussed above, progressive molds may be formed by a variety ofprocesses known in the art. Mold 40 in FIG. 3 depicts a mold that hasbeen made according to any one of these processes. The mold 40 includesa back face 5 shaped to include a section adapted to cast a near visioncorrection zone 44, a section adapted to cast a far vision correctionzone 46, and a section adapted to cast a vision transition zone 43.FIGS. 4 and 5 represent enlarged portions 30 and 31 in FIG. 3, with theexception that FIGS. 4 and 5 do not include sections 50-53 (seedescription below).

Mold 40 may not initially include a flat surface or band such as band 7in FIG. 1. To provide such a band, mold 40 is typically tapered byapplying it to a cutting or grinding surface. This surface may besubstantially flat or slightly convex. This tapering removes mold 40sections 50 and 51, thus forming band 7 with a width 8 near the bottomof the mold and width 10 near the top of the mold. As shown in FIG. 3,in a typical process mold 40 is applied substantially evenly to thecutting or grinding surface. Typically sections 50 and 51 areapproximately the same size. If sections 50 and 51 are cut from mold 40,then line 12 in FIG. 3 represents the point at which the sections 50 and51 are severed from the mold 40. If sections 50 and 51 are ground frommold 40, then line 12 in FIG. 3 represents the point to which mold 40 isground to remove sections 50 and 51. It is to be understood thatsections 50 and 51 in FIG. 3 are only schematically drawn, and the sizeof these sections can vary depending on the width of band 7 and otherfactors. Mold 40 could be tapered according to lines parallel and to theright or left of line 12, thereby making sections 50 and 51 smaller orlarger. For instance, lines 12' and 12" in FIGS. 4-5 represent linesparallel and to the right of line 12 in FIG. 3.

Tapering mold 40 as described above provides the mold 40 with a band 7which can act as a sealing surface when the mold 40 is used to form amold cavity to cast lenses. This tapering, however, does not reduce thevertical prism effects described above.

The method of the invention involves shaping the back face 5 of theprogressive mold 40 so that vertical prism effects are reduced. Themethod of the invention may be accomplished by tapering the edges of theback face 5 so that the average thickness of near vision zones in lensescast by the mold 40 is thinned. This may be accomplished bydisproportionately tapering the edges of the back face 5 of the mold 40so that the amount of the back face 5 removed from the edges of the backface 5 adjacent the near vision correction zone 44 is larger than theamount of back face 5 removed from the edges of the back face 5 adjacentthe far vision correction zone 46. The tapering has a more dramaticeffect on the thickness of the edges of lenses cast by the mold versusthe effect of thickness near the center of lenses cast by the mold. Inother words, the tapering thins the resultant lenses in varying amounts,with the thinning being greatest near the edges of the near vision zone.

As shown in FIGS. 3-5, the method of the invention may be accomplishedby shaping the back face 5 of the mold 40 by tapering it at an angle 17such that the amount of back face 5 proximate to the near vision zone 4which is tapered is greater than the amount of back face 5 proximate tothe far vision correction zone 6 which is tapered. As shown in FIG. 3,line 14 represents the point to which mold 40 is tapered to removesections 52 and 53, thereby forming band 7 with a width 9 near thebottom of the mold and width 11 near the top of the mold. As shown inFIG. 3, in the method of the invention the mold 40 is applied unevenlyto the tapering surface. Thus sections 52 and 53 are not the same size.If sections 52 and 53 are cut from mold 40, then line 14 in FIGS. 3-5represents the point at which the sections 52 and 53 are severed fromthe mold 40. If sections 52 and 53 are ground from mold 40, then line 14in FIGS. 3-5 represents the point to which mold 40 is ground to removesections 52 and 53. It is to be understood that sections 52 and 53 inFIGS. 3-5 are only schematically drawn, and the size of these sectionscan vary depending on the width of band 7 desired and other factors. Forinstance, mold 40 could be tapered according to lines parallel and tothe right or left of line 14, thereby making sections 52 and 53 smalleror larger.

As shown in FIG. 4, when section 52 is removed, part of the near visioncorrection zone 44 of the back face 5 may also be removed. Thus, asshown in FIG. 4, width 8 usually changes to width 9 when the mold 40 istapered according to the method of the invention. Depending on systemvariables (e.g. mold thickness, angle 17, where line 14 is placed,etc.), increasing the size of section 52 thus usually increases thewidth of band 7. It is to be understood, however, that in somecircumstances increasing the size of section 52 may not affect, or evendecrease, the width of band 7 (if, for example, the mold had a constantthickness, and angle 17 was significantly decreased at the same timethat line 14 was moved to the left on FIG. 3). Generally speaking, thesize of section 52 can be increased by increasing angle 17, and/or bymoving line 14 to the left on FIGS. 3-5.

As shown in FIG. 5, when section 53 is removed, part of the far visioncorrection zone 46 of the back face 5 may also be removed. As such,width 10 may be increased or decreased to width 11. In FIG. 5 width 11is shown as smaller than width 10, however it is to be understood thatwidth 11 can also be the same as or larger than width 10 depending onsystem variables.

Generally speaking, as width 9 is increased, larger portions of the nearvision correction zone 44 are removed. Similarly, as width 11 isincreased, larger portions of the far vision correction zone 46 areremoved. Widths 9 and 11 should be small enough to prevent an excessiveamount of near vision correction zone 44 or far vision correction zone46 being removed. If too much near vision zone 44 or far visioncorrection zone 46 is removed, then the mold will produce a lens witheither a near or far vision correction zone that is unduly small andcommercially unsalable. Correspondingly, if widths 9 and 11 are toosmall, then gaskets used to cast lenses will not seal against band 7 tohold liquid lens-forming compositions in mold cavities formed in part bythe mold and such gaskets. In a preferred embodiment, widths 9 and 11are in the range of about 1 to 10 millimeters, and more preferably isabout 2 to 8 millimeters, and more preferably still is about 3 to 6millimeters.

As shown in FIG. 4, after section 52 is removed, the portion of band 7proximate to the near vision correction zone is angled inward andtowards the center 21 at angle 17. As shown in FIG. 5, after section 53is removed, the portion of band 7 proximate to the far vision correctionzone is angled inward and away from the center 21 at angle 17. Angle 17is preferably between 0.25 and 10 degrees, more preferably between 0.25and 5 degrees, and more preferably still between 0.25 and 2 degrees. Inpreferred embodiments, the angle 17 was 0.5, 0.75, and 1.0 degrees. Theamount of angle 17 is limited by desired limits of widths 9 and 11. Forinstance, as angle 17 increases, then width 9 usually also increases.

FIG. 6A depicts a lens 60 that has been cast by a mold that was notcorrected according to the method of the invention. As shown in FIG. 6A,the lens 60 has a near vision correction zone thickness 62 that isthicker than the far vision zone thickness 64. As a result, verticalimage displacement 65 (i.e. vertical prism effects) resulted. FIG. 6Bdepicts the same prescription lens from FIG. 6A wherein that lens nowhas been cast by a mold that was corrected according to the method ofthe invention. As shown in FIG. 6B, the lens 66 has a near visioncorrection zone thickness 68 that is still thicker than the far visioncorrection zone thickness 70, however thickness 68 is now closer tothickness 70. As a result, vertical image displacement 67 (i.e. verticalprism effects) resulted. As shown in FIGS. 6A and 6B, vertical imagedisplacement 67 is less than vertical image displacement 65, indicatingthat the vertical prism effects in lens 66 are less than they are inlens 60.

EXAMPLES

Glass progressive molds that are prepared using a slump molding processare available from Shamir Optical Industries (Kibbutz Shamir, UpperGalilee, Israel). Glass progressive molds received from Shamir weretested to determine the amount of vertical prism effects present inlenses cast using such molds. These molds were disk-shaped, and wereapproximately 80 millimeters in diameter and 3.2-4.0 millimeters thick.The molds were used to cast lenses with ultraviolet light as describedin the U.S. patent applications entitled "Apparatus and Process For LensCuring and Coating" by Buazza et al., filed the same day as thisapplication, and which is a continuation-in-part of U.S. applicationSer. No. 800,561, filed Dec. 6, 1991. It is possible to cast lensesusing other apparatus and methods known in the art. Various molds weretested both before and after they were prepared according to the methodof the invention.

Table 1 summarizes the results. Column 1 in Table 1 shows the moldreference base, in diopters ("D"), of the far vision correction zone inlenses prepared with various molds (hereinafter "mold reference base").Column 2 in Table 1 shows the amount of power added to near visioncorrection zones over and above the mold reference bases (hereinafter"add power"). Column 3 of Table 1 shows the amount of vertical prism, inprism diopters ("ΔD"), of lenses prepared with molds that wereuncorrected according to the method of the invention. Vertical prismeffects were measured at the geometric center of each lens. Column 4 inTable 1 shows the amount of vertical prism of lenses prepared with moldsthat were corrected according to the method of the invention. Again,vertical prism effects were measured at the geometric center of eachlens. Column 5 in Table 1 shows width 9 of the corrected molds inmillimeters ("mm").

                  TABLE 1                                                         ______________________________________                                                                     4                                                                  3          Prism in                                         1                 Prism in Lens                                                                            Lens Made                                                                             5                                        Mold    2         Made by    by      Width 9 of                               Reference                                                                             Add Power Uncorrected                                                                              Corrected                                                                             Corrected                                Base (D)                                                                              (D)       Mold (ΔD)                                                                          Mold (ΔD)                                                                       Molds (mm)                               ______________________________________                                        +6.00   +1.00     0.75        .75    N/A                                      +6.00   +1.25     0.87        .87    N/A                                      +6.00   +1.50     1.00       1.00    N/A                                      +6.00   +1.75     1.25       1.25    N/A                                      +6.00   +2.00     1.50       1.50    N/A                                      +6.00   +2.25     1.62       1.25    4.0                                      +6.00   +2.50     1.75       1.37    4.5                                      +6.00   +2.75     2.00       1.50    5.0                                      +6.00   +3.00     2.25       1.50    5.5                                      +4.00   +1.00     0.75        .75    N/A                                      +4.00   +1.25     0.87        .87    N/A                                      +4.00   +1.50     1.00       1.00    N/A                                      +4.00   +1.75     1.25       1.25    N/A                                      +4.00   +2.00     1.50       1.50    N/A                                      +4.00   +2.25     1.62        .75    5.0                                      +4.00   +2.50     1.75       1.00    5.5                                      +4.00   +2.75     2.00       1.25    6.0                                      +4.00   +3.00     2.25       1.50    6.5                                      +3.00   +1.00     0.75        .75    N/A                                      +3.00   +1.25     0.87        .87    N/A                                      +3.00   +2.00     1.50       1.50    N/A                                      +3.00   +2.25     1.62       1.00    6.0                                      +3.00   +2.50     1.75       1.0     7.0                                      +3.00   +2.75     2.00       1.12    7.5                                      +3.00   +3.00     2.25       1.12    8.0                                      ______________________________________                                    

As shown in Column 4, molds that were corrected according to the methodof the invention produced lenses with reduced vertical prism effects inthe far vision correction zones of these lenses. The amount of reductiontended to increase as the angle of correction increased. In addition, asthe angle of correction was increased, then the width 9 increased, thusreducing the amount of near vision correction zone. Thus the amount ofthe angle of correction is limited by the amount of near visioncorrection zone in resultant lenses that can be acceptably reduced.

Optimally lens vertical prism effects are zero. If the vertical prismeffects in a single lens is greater than 1.5 ΔD, then wearers may sufferheadaches, nausea, and loss of visual function. As shown in Table 1,unacceptably high vertical prism effects (i.e., greater than about 1.5ΔD) tend to be produced in higher add power lenses made with uncorrectedmolds. Thus some correction of vertical prism effects was necessary inhigher add power lenses. The term "N/A" in Column 5 means that nocorrection was applied since the molds already produced lenses with lessthan 1.5 ΔD of vertical prism effects (thus width 9 did not differ fromwidth 8).

If the vertical prism imbalance between lenses in a pair of glasses isgreater than 0.5 ΔD, then wearers may suffer headaches and nausea.Advantageously the amount of vertical prism effects differ no more thanabout 0.33 prism diopter between right and left lenses in a pair ofglasses (ANSI Z80.1 - Apr. 15, 1991). More preferably, this differenceis less than about 0.12 prism diopter. If the vertical prism effectsdifference between lenses in a pair of glasses is too much, then thewearer may also tend to experience diplopia (i.e. see double) becausethe wearer tends to have difficulty merging images. Thus the method ofthe invention can be used to optimize vertical prism effects.

The results shown in Columns 4-5 in Table 1 were obtained empirically byexperimenting with various angles of correction applied to mold holdingdevices. The holding device was locked into place when a certain angleof correction was applied to it such that molds prepared with the devicecast lenses with less than about 1.5 ΔD with a given mold reference baseand add power combination. The actual angles of correction for eachlocked position was not recorded, but is estimated to be preferablyabout 0-2 degrees, more preferably about 0.25 to 1.0 degrees, and morepreferably still 0.5 degrees. Different mold holding devices were lockedinto place for different mold reference base and add power combinations.It has been found that it is possible to prepare a locked holding devicefor each mold reference base and add power combination. In this mannerdifferent holding devices can be used for different mold reference baseand add power combinations instead of using the same holding device witha varied angle of correction.

Further modifications and alternative embodiments of various aspects ofthe invention will be apparent to those skilled in the art in view ofthis description. Accordingly, this description is to be construed asillustrative only and is for the purpose of teaching those skilled inthe art the general manner of carrying out the invention. It is to beunderstood that the forms of the invention shown and described hereinare to be taken as the presently preferred embodiments. Elements andmaterials may be substituted for those illustrated and described herein,parts and processes may be reversed, and certain features of theinvention may be utilized independently, all as would be apparent to oneskilled in the art after having the benefit of this description of theinvention. Changes may be made in the elements described herein or inthe steps or in the sequence of steps of the methods described hereinwithout departing from the spirit and scope of the invention asdescribed in the following claims.

We claim:
 1. A cast prescription plastic progressive eyeglass lenscomprising a front face, a back face, a near vision correction zone, afar vision correction zone, and a vision transition zone between thenear vision correction zone and the far vision correction zone, the lensbeing prescription-cast and having a vertical prism of less than about1.5 prism diopter, and the lens having a from face cast from and shapedby the back face of a mold, the back face of the mold including (1) acentrally located concave surface having a near vision correction zoneadapted to cast the near vision correction zone of the lens, a farvision correction zone adapted to cast the far vision correction zone ofthe lens, and a vision transition zone adapted to cast the visiontransition zone of the lens, and (2) an annular surface which tapers atan angle from the edges of the back face of the mold to the edges of theconcave surface in the back face of the mold.
 2. The plastic progressiveeyeglass lens as recited in claim 1 wherein the lens is prescriptioncast to an add power of greater than about 2 diopter.
 3. The plasticprogressive eyeglass lens as recited in claim 1, wherein the width ofthe annular surface of the back face is between about 0.5 and 10.0millimeters when measured from the edges of the back face to the edgesof the concave surface.
 4. The plastic progressive eyeglass lens asrecited in claim 1, wherein the width of the annular surface of the backface is between about 2.0 and 8.0 millimeters when measured from theedges of the back face to the edges of the concave surface.
 5. Theplastic progressive eyeglass lens as recited in claim 1, wherein theangle of the taper from the edges of the back face to the edges of theconcave surface is about 0.25 to 10.0 degrees.
 6. The plasticprogressive eyeglass lens as recited in claim 1, wherein the angle ofthe taper from the edges of the back face to the edges of the concavesurface is about 0.25 to 5.0 degrees.
 7. A plastic progressive eyeglasslens as recited in claim 1 wherein the lens is cast by directingultraviolet light towards the from face of the mold.
 8. The plasticprogressive eyeglass lens as recited in claim 1 wherein the mold isshaped to reduce vertical prism effects otherwise associated in a lenscast by the mold.
 9. The plastic progressive eyeglass lens as recited inclaim 8 wherein the mold is shaped by tapering the edges of the backface of the mold such that the average thickness of the near visioncorrection zone of the lens is thinned by an amount that is more than anamount that the average thickness of the far vision correction zone isthinned.
 10. The plastic progressive eyeglass lens as recited in claim9, wherein the tapering is disproportionate so that the amount of theback face removed from the edges of the back face proximate to the nearvision correction zone is larger than the amount of back face removedfrom edges of the back face proximate the far vision correction zone.11. The plastic progressive eyeglass lens as recited in claim 9, whereinthe tapering is accomplished by grinding or cutting the mold.
 12. Theplastic progressive eyeglass lens as recited in claim 9, wherein themold is shaped such that the back face comprises a substantially flatband which connects from the edges of the back face to the edges of theconcave surface in the back face.
 13. The plastic progressive eyeglasslens as recited in claim 12, wherein the width of the band is betweenabout 0.5 and 10.0 millimeters when measured from the edges of the backface to the edges of the concave surface.
 14. The plastic progressiveeyeglass lens as recited in claim 12, wherein the width of the band isbetween about 2.0 and 8.0 millimeters when measured from the edges ofthe back face to the edges of the concave surface.
 15. The plasticprogressive eyeglass lens as recited in claim 12, wherein the concavesurface has a center, and wherein the band adjacent the near visioncorrection zone is angled inward and towards the center of the concavesurface.
 16. The plastic progressive eyeglass lens as recited in claim15 wherein the angle of the taper is about 0.25 to 10.0 degrees.
 17. Theplastic progressive eyeglass lens as recited in claim 15 wherein theangle of the taper is about 0.25 to 5.0 degrees.
 18. The plasticprogressive eyeglass lens as recited in claim 1 wherein the lens isformed by placing a polymerizable lens forming material in a mold cavityat least partially defined in part between the back face of the mold,and then curing the polymerizable lens forming material by directingultraviolet light towards the lens forming material.